The ability to detect and identify unknown gases is increasingly important for homeland defense and other safety reasons. For example, the Occupational Safety and Health Administration (OSHA) requires that a four-gas meter be carried by all personnel working in a confined space. These meters are also carried by all hazardous material (HAZMAT) teams. While these meters can alert an operator to the presence of a gas, the meters cannot identify the components of the gas. It is advantageous to be able to differentiate and identify unknown substances to determine the type and extent of hazard present in an environment. Gas monitoring instruments are useful in a variety of situations. For example, a gas monitoring system can protect personnel from otherwise undetectable hazards that may exist in workplace environments. Gas monitoring systems can also protect and assist a first responder in situations where hazardous material cleanup may be necessary.
Various methods for identifying unknown chemicals are known. These methods include, for example, various forms of vibrational spectroscopy such as infrared spectroscopy, nuclear magnetic resonance spectroscopy, gas chromatography, mass spectrometry, ion mobility spectrometry, and x-ray crystallography. Each of these methods has its own benefits and drawbacks. Often the type of method used to identify an unknown sample will depend on the form of the sample and the information which needs to be identified.
Infrared (IR) spectroscopy is particularly useful for the identification of unknown gases. Spectroscopic analysis using radiant energy in the infrared region of the electromagnetic radiation spectrum is a primary technique for chemical analysis of molecular compounds. The infrared spectral region extends from 0.7 to 250 micrometers, wherein the mid-IR region is generally considered to cover the region from about 2.5 to about 25 micrometers, which is commonly used for molecular vibrational spectroscopy. The mid-IR region of the spectrum arises from the fundamental movement of chemical bonds in molecules. When a beam of infrared energy is passed through an unknown sample, a spectrum, or characteristic fingerprint, of the molecules making up the sample is obtained. The unique spectrum obtained allows the components of the sample to be identified using a fundamental understanding of vibrational spectroscopy by comparison with a library of known compounds.
Fourier Transform infrared (FTIR) spectroscopy is especially suitable for quick identification of unknown samples due to its high sensitivity and rapid operational speed. However, FTIR suffers significant limitations due to technical and size limitations inherent in the detectors used in FTIR instruments making the instruments difficult to use in a field setting. These limitations are more pronounced when a sample is gaseous because the concentration of unknown in the sample is typically lower in a gas than in a liquid or solid sample. Prior approaches to increasing detection sensitivity include treatment of the sample or sample collection procedure and modifications to an instrument's detector.
One approach to modification of the instrument's detection system includes the use of long path cells. Long path gas cells can be sensitive enough to not require pre-concentration of a sample, but such devices generally require liquid nitrogen cooled detectors which are practicable only in a laboratory setting. Additionally, the volume and mass of these cells is large, requiring a larger sample, which must be collected and transported to a laboratory for analysis.
Another approach to analyzing gas samples involves absorption of a sample onto a sorbent followed by thermal desorption of the sample into a hollow wave guide acting as a gas cell for FTIR. See, Pogodina et al, Anal. Chem. 76, 464-68 (2004). Because a waveguide transmits less energy than a standard long path gas cell, this method also requires a research grade FTIR outfitted with a liquid nitrogen cooled detector.
Despite the various methods and techniques used in developing gas detection and identification systems, current systems are often expensive and bulky, which severely limits their usefulness in the field. Furthermore, current systems and methods do not allow for sample collection followed by decontamination of the sample collection device either prior to or immediately after sample analysis and prior to removal from the site of potential contamination to thereby prevent contamination of other locations and personnel.